gspawn-win32: Fix multiple definition error with a static build
gspawn-win32: Fix multiple definition error with a static build

gspawn-win32.c gets included by gspawn-win32-helper.c and in case
of a static build the definitions there clash with the ones from
libglib. Fixed by not compiling the ABI comapt code in case
GSPAWN_HELPER is defined.

I missed this issue in commit 23dffdd949eb1c

https://bugzilla.gnome.org/show_bug.cgi?id=780634

johne53/MB3Glib,johne53/MB3Glib,johne53/MB3Glib,endlessm/glib,endlessm/glib,johne53/MB3Glib,endlessm/glib,johne53/MB3Glib,johne53/MB3Glib,endlessm/glib,endlessm/glib